tests: drivers: dma: Increase test coverage for RP2350A

Increase test coverage for Raspberry Pi's SoCs. Use the `socs` folder
rather than `boards` to enable these tests to run on any boards with the
same SoCs.

Signed-off-by: Andrew Featherstone <andrew.featherstone@gmail.com>

Author: ajf58

tests/drivers/dma/chan_blen_transfer/socs/rp2350a_m33.overlay

@@ -0,0 +1,8 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Copyright (c) 2024 Andrew Featherstone <andrew.featherstone@gmail.com>
+ */
+
+/* RP2350A is compatible with the RP2040, so reuse. */
+#include "rp2040.overlay"